Specially designed ballasts for dimming fluorescent lamps are presently available and well known in the art. Many ballasts achieve dimming ratios of better than 1000:1. However, these systems operate as phase control systems, meaning that a large spike of current must flow along the power wiring to reignite the lamp when the lamp has not conducted for a period greater than about 1 millisecond. The resulting current and voltage spikes appear in the power line and may radiate from the lamps and building wiring causing electromagnetic interference (EMI) which may affect the operation of sensitive electronic equipment. It is especially important to keep transients off of the building wiring because it behaves like a transmission antenna and thus extends the range over which the EMI radiates.
Using specially designed dimming ballasts to replace one of the over 600 million U.S. installed 40 watt fluorescent lamps supplied by a non-dimming ballast in order to adapt an installation to a dimming system can be very expensive. Furthermore, these special dimming ballasts typically power only one lamp while most existing fluorescent lighting installations contain four or more lamps. Thus, it is desirable to be able to adapt the conventional non-dimming ballasts to a dimming configuration.